Pour a small amount of pure old liquid chlorine bleach (2 to 3 tablespoons) into one cup. Leave it full strength. Swirl it around so the bleach coats the entire inner surface of the cup.......then pour it into the next stained cup and do the same. Let the treated cups set for 5 or 10 minutes. Then just wash normally with soap and water and rinse,stains gone! I always put a "small" splash of liquid bleach in my sink each time I fill it to wash dishes. I don't take a chance on bacteria hanging around on my dishcloths or sponges!